A voice coil motor, as is well known, comprises cylindrical magnets, an outer yoke and an inner yoke. The voice coil motor also comprises a cylindrical coil that is arranged in an annular space in which magnetic field is intensively generated, such that the coil is driven in an axial direction when current is supplied to the coil. Direct-acting (linear driving) servo valves using such voice coil motors are known to public. The direct-acting servo valve drives a spool placed in a valve body in an axial direction by means of the voice coil motor, and switches the communication between ports provided on the valve body by opening and closing the ports. As a result, the flow rate of fluid flowing through the ports is regulated, and velocity, position or forces of a loading device such as a hydraulic cylinder or a fluid motor are controlled. Servo valves are used in various machines and equipment such as pressing machines, machine tools, steel manufacturing facilities, airplanes or fatigue testing machines which require quick response as well as relatively large power.
The document D1 discloses a structure in which voice coil motors including yokes having E-shape in their vertical sections (see FIG. 3 in the document D1) are placed on both axial ends of a spool so as to enhance the axial driving force to the spool. By providing two voice coil motors, the driving force to the spool may be doubled. However, because total mass of these voice coil motors is also doubled, this case may result in a deteriorated responsiveness in some degree. Further, because two amplifiers are necessary for driving the voice coil motors, a problem of increased cost in addition to the problem of increased heat generation caused by the voice coil motors may occur.
It is possible to provide a voice coil motor as disclosed in the documents D2 and D3 on one end of the spool. Such a voice coil motor uses a magnetic circuit that is generally called as “Dual Halbach Magnet Array” by which driving magnetic field of the voice coil motor can be increased so as to generate larger driving force to drive the spool in an axial direction. In these cases, only a single voice coil motor is necessary, leading to improved speed response, and furthermore, only a single amplifier being required. The structure of the voice coil motor using a Dual Halbach Magnet Array, which enhances interacting magnetic field for the coil so as to improve the drive efficiency, is already well known by the documents D4, D5 and D6 in addition to the documents D2 and D3.
A voice coil motor using a Dual Halbach Magnet Array is shown in FIG. 11 in principle. In a Dual Halbach Magnet Array, a cylindrical coil 120 is arranged in an annular space between an outer magnet array 100 and an inner magnet array 110, such that the coil 120 becomes movable in axial directions to the left or to the right depending on the alternative direction of the current supplied to the coil 120. The outer magnet array 100 is configured by axially arranging ring-shaped radially magnetized magnets 101 and ring-shaped axially magnetized magnets 102 to be adjacent to each other in a manner that magnetic poles of the radially magnetized magnets 101 and the axially magnetized magnets 102 are rotated by 90 degrees to each other in a cross section including center axes. An inner magnet array 110 is configured by axially arranging ring-shaped radially magnetized magnets 111 having the same direction of magnetic poles as the radially magnetized magnets 101 of the outer magnet array 100, and ring-shaped axially magnetized magnets 112 whose magnetic poles have reverse directions with respect to the axially magnetized magnets 102 of the outer magnet array 100. An outer periphery of the outer magnet array 100 is supported by an outer cylindrical member 105, and an inner periphery of the inner magnet array 110 is supported by an inner cylindrical member 115. The reference numeral 121 denotes a coil bobbin, whereas the reference numerals 125 and 126 denote side plates.
The ring-shaped axially magnetized magnets 102 and 112 can be manufactured easily by the conventional technology. However, it is difficult to manufacture the ring-shaped magnets 101 and 111 which are radially magnetized in a favorable manner. This is because it is difficult to magnetize a ring-shaped magnet such that it has effective and high magnetic flux density in its radial direction due to the difference between the area of the inner peripheral surface and that of the outer peripheral surface of the ring-shaped magnet. Particularly, in case there is a large difference between the inner and outer diameters, it is more difficult to radially magnetize the ring-shaped magnet effectively.
As one way to cope with this difficulty, a magnetic ring (not magnetized yet) may be divided in its circumferential direction into a plurality of parts, and then each of the parts may be magnetized in its radial direction. Thereafter, all of the radially magnetized parts may be combined into a ring to form a radially magnetized magnet. It is relatively easy to radially magnetize the parts individually even with the conventional technology. In this way, if the magnetized parts are combined into a ring, it may be possible to manufacture the radially magnetized magnet having effective and high magnetic flux density in its radial direction.
An adhesive may be considered to be used in order to combine the plurality of split magnets into a ring-shape. However, since inner and outer peripheries of the split magnets adjacent in their circumferential directions have the same polarity respectively, repulsive force always acts therebetween. Thus, the adhesive tends to be broken due to disturbances such as changes in ambient temperature or vibration, so that the split magnets may possibly pop outwardly in the radial direction. Consequently, the radially magnetized magnets may not be able to work anymore.
Split-type radially magnetized magnet may be applicable for both of the outer magnet array 100 and the inner magnet array 110. Regarding the outer magnet array 100, the split magnets will not pop outwardly since their outer peripheral surfaces are surrounded by the outer cylindrical member 105. Further, since both circumferential sides of each of the split magnets have radially inclined surfaces, those inclined surfaces will act as wedges so as to prevent the split magnets from popping inwardly. Regarding the inner magnet array 110, the split magnets cannot pop inwardly since the inner peripheral surface thereof are supported by the inner cylindrical member 115. However, the split magnets cannot be prevented from popping outwardly because their outer peripheral surfaces are not supported in surrounding manner. If a split magnet pops outwardly, it may interfere with the coil, resulting in a failure in the operation of the voice coil motor.